First Law of Thermodynamics – Flashcards

The formula for pV work against constant pressure is w = -pV. When the system expands, V is positive and w is negative. This means that the energy decreases during expansion. When the system is compressed, V is negative and w is positive, which means that the system gains energy during compression.

The heat capacity, C, of an object tells you how much heat, q, is needed to produce a given rise in temperature, i.e., q = C(Delta)T. For a pure substance, we can us the molar or specific heat capacities and the amount of the substance to calculate the heat capacity.

One reason that heat capacities can differ is that a substance may use some of the energy transferred as heat to change the potential energy of the particles that compose it. It can do this by, for example, changing the average distance between the particles. Since some of the added heat is put into potential energy, less can go into the kinetic energy of the particles, so that the change in temperature is smaller. The differences between substances can be accounted for by the difference in

Enthalpy is the name we give to the state function that represents the energy transferred as heat at constant pressure. The change in enthalpy is given by:

(Delta)H = (Delta)E + p(Delta)V

We can determine (Delta)E and (Delta)H for ANY process since (Delta)E is just the sum of the work and the heat and (Delta)H is given by the formula in the last question. The point to note is that the relation between these two quantities and the heat differs for different processes. For an isochoric process, (Delta)E is equal to the heat itself, and (Delta)H has no special meaning. For an isobaric process, (Delta)E is now the sum of the work and the heat and (Delta)H is equal to the heat itself.

In simple mathematical terms, the heat capacity can be infinite when we can add energy as heat and not produce a change in temperature. This implies that all of the energy is being used to change the potential energy

If you have a series of reactions that add up to give an overall reaction, then the standard enthalpy change for the overall reaction will equal the sum of the standard enthalpy changes for the individual reactions.

In simple terms, the system is that thing or set of things that we are describing and the surroundings is everything else. The combination of system and surroundings is called the universe. The nature of the separation between system and surroundings, i.e., the wall, controls how energy and matter can be exchanged between system and surroundings.